Carl & Jerry: Feedback
May 1956 Popular Electronics

Wax nostalgic about and learn from the history of early electronics. See articles
from Popular Electronics,
published October 1954 - April 1985. All copyrights are hereby acknowledged.

In this episode of John T. Frye's "Carl & Jerry" series,
the intrepid pair of teenage electronics hobbyists and Ham radio
operators are experimenting with an audio amplifier rig that
uses a parabolic dish for concentrating sound waves at a focal
point where they have a microphone mounted. Aside from picking
up bird noises and a neighbor lady scolding her husband for
not properly washing the windows during a round of Spring cleaning,
Carl imposes upon Jerry for a lesson in feedback techniques
- both positive and negative - and the reasons one is preferred
over the other. The story winds up with a clever double entendre
comment referring to 'osculation.'

Carl & Jerry: Feedback

By John T. Frye

You
might have thought - if you were a careless observer and didn't
knew the boys very well - that Carl and Jerry were doing nothing.
It must be admitted that they looked idle as they sprawled on
the turf of Jerry's back yard under the relaxing rays of the
warm May sun. However, you had only to look a little closer
to notice the slowly revolving reels on the tape recorder beside
them and to see the cord leading from it to a slender microphone
mounted in the center of a metal object shaped like a huge shallow
dish some three feet in diameter. This was propped up on edge
so that its concave side carrying the microphone faced away
from the boys toward the hedge separating Jerry's yard from
the one next door.

"I think that's a lot of stuff about that king-size popcorn
bowl picking up sounds we can't hear," Carl muttered in low
tones with a somewhat disparaging glance at it.

"Have it your way," Jerry answered, "but when we play back
the tape, you'll hear these birds that are playing around in
the hedge now cheeping and twittering as though they were right
in front of the microphone. That parabolic reflector focuses
the sound waves on the microphone the same way the concave mirrors
we play around with in the physics lab at school focus light
rays down to a single bright spot. People who collect bird calls
and insect sounds use this technique all the time. Just keep
your voice down so you don't scare the birds and so the microphone
doesn't pick it up."

"Mrs. Selden is the one who ought to keep her voice down,"
Carl remarked, as the shrill complaining of the woman next door
came through the hedge.

"They're taking out the storm sash and washing the windows,"
Jerry reported, after rising to his knees so that he could see
over the hedge.

"Hew come she bends Mr. Selden's ear that way all the time?"
Carl asked.

"Habit, I guess. Leastways, that's what Mom thinks. She says
Mrs. Selden has been scolding so long she doesn't know she's
doing it," Jerry explained with a yawn as he stretched out on
the grass again.

"Hey, Jer," Carl said lazily, toying with the rubbery stem
of a plucked dandelion, "how's about briefing me a little on
negative feedback while we're eavesdropping on the birds? I'm
cooking up a new speech amplifier for my ham rig, and I don't
know whether to use feedback or not."

"Hokey-dokey," Jerry agreed. "To begin with, feedback is
simply the taking of some of the output of a device and feeding
it back into the input. As far as an amplifier is concerned,
feedback comes in two different flavors: if the portion of the
output signal fed back is 'phased' or timed so that it aids
or increases the swing of the input signal, it's called 'positive'
or 'regenerative' feedback. Positive feedback increases the
amplification and results in greater output. But an amplifier
with positive feedback has a strong tendency to favor one frequency
over the others and so produce harmonic distortion and non-linear
amplification. Worse yet, when enough positive feedback is applied,
the circuit breaks into oscillation at essentially this favored
frequency, and the circuit then becomes useless as an amplifier.
Generally speaking, positive feedback is a darned nuisance as
an audio amplifier - it leads to howling, motorboating, and
poor performance; but don't forget that when it comes to oscillators,
we must have positive feedback or we don't have any oscillation.

"'Negative,' 'degenerative,' or 'inverse' feedback is phased
so that the energy returned to the input circuit from the output
actually opposes the signal voltage acting on the grid and reduces
the amplification. This contrariwise relationship between the
plate and grid circuits works to advantage, for any hum or noise
or distortion generated in. the plate circuit tends to 'buck'
itself out. The distorting 'zig' in the plate circuit produces
an opposite-going 'zag' in the controlling grid voltage that
sends through a correcting signal to help iron out the distortion
in the output."

... The huge shallow disc was propped up
on edge so that its concave side faced toward the hedge separating
Jerry's yard from the one next door ...

"Hey, that's pretty cute: it's as though the circuit were
correcting all its own faults!"

"Yes, negative feedback has several advantages. It reduces
output circuit hum, noise, and harmonic distortion by the same
percentage that it reduces the gain. When it's applied as it
should be, negative feedback evens up the amplification given
to different frequencies and makes an amplifier more nearly
'flat' in its frequency response."

"Negative feedback seems to have everything. What's the catch?"
Carl wanted to know.

"The only catch is that it reduces the gain by the same amount
that it reduces the distortion. It can only be applied when
you have sufficient surplus of gain to sacrifice amplification
in order to obtain the other advantages."

"How do you put negative feedback into a circuit?" was Carl's
next question.

"An easy way to do it with a single-tube amplifier is simply
to leave the cathode resistor unbypassed. In this case, the
cathode resistor becomes a part of the plate load, and a portion
of the plate signal voltage appears between the cathode and
ground. The voltage across the cathode resistor also appears
in series with the signal voltage on the grid, but it's of a
polarity which opposes that signal voltage. Take a 'ferinstance.'
Say that the signal swings the negatively biased grid less negative.
This increases both plate and cathode current. Increased cathode
current increases the voltage drop across the cathode resistor,
making the cathode more positive with respect to ground. Since
the grid is connected to ground, it also makes the cathode more
positive with respect to the grid, or the grid more negative
with respect to the cathode. This last action opposes the original
signal voltage that was driving the grid less negative."

"The amount of feedback in such a case would be determined
by the ratio of the cathode resistor to the plate load resistor,
I suppose."

"That's right. An interesting example of 100% negative feedback
occurs in the cathode follower circuit in which the plate is
grounded, as far as signal voltage is concerned, and the cathode
resistor is the entire plate load. In this case, the voltage
gain or amplification of the stage is reduced to less than one,
but distortion is practically nil."

"I suppose there are other ways of introducing negative feedback."

"Oh, sure.' Quite often a lead is run from one side of a
speaker voice coil back to the grid or cathode of a preceding
stage. By selecting the proper end of the output transformer
secondary, you can get a voltage that will constitute 'negative
feedback' for any preceding stage. Remember that every time
a signal passes through a tube it undergoes a 180° phase
shift; so a voltage that would be 'negative feedback' at the
grid of one tube would be 'positive feedback' at the grid of
a preceding or following stage."

"Then a feedback loop may embrace more than just one tube."

"That's right. It's quite common to feed back for two or
three stages."

"I notice you speak about negative feedback being applied
to a 'device: Did you mean to say that?"

"Yes. Feedback is found in a lot more places than audio amplifiers.
For example, in a public address system, when the volume is
boosted too high, you get positive acoustic feedback from the
speaker to the microphone that results in a howl or oscillation.
It's interesting to note, incidentally, that this howl usually
occurs on a particular note for a given system. Remember we
said that positive feedback favored one frequency?

"A fine example of negative feedback applied to a mechanical
system," continued Jerry, after taking a deep breath, "is in
the governor of a steam engine. This governor consists of two
metal balls attached by hinged rods to a vertical shaft that
is rotated by the steam engine. As the balls swing toward or
away from the shaft, they control a valve that regulates the
amount of steam admitted to the engine. When they're resting
next to the shaft, the valve is wide open; and the farther they
swing out, the more this valve closes. When the engine tries
to speed up, the vertical shaft is rotated faster and centrifugal
force causes the balls to swing out, cutting down on the steam
and slowing down the engine. If the application of a heavy load
reduces the speed of the engine, the balls swing in and open
the valve, which restores the speed.

"Feedback even plays an important part in our physical actions.
For example, notice what happens when I decide to pick up that
twig. My brain sends a message to my hand that starts it moving
toward the twig. As my hand moves, my eye keeps measuring the
distance that still separates my hand from the little branch
and constantly reports this information back to the brain. As
the distance grows less and less, the information fed back is
acted upon to cause my hand to slow down and finally stop directly
over the twig."

"In fact," Jerry concluded, as he rolled over to stop the
tape recorder and start rewinding the tape, "feedback plays
a most important part in electronic brains, guided missiles,
and so on. In all these devices, the data, direction, or movement
is constantly being sampled and tested and fed back to the controlling
mechanism to answer its unceasing need to know 'How am I doing?'"

As he finished speaking, he started the tape playing through.
the recorder. At first, the only sound was that of the birds
chirping away with amazing volume - and lifelike clarity. They
sounded as though they might have been perched right on the
microphone. Suddenly, though, the shrill complaining voice of
Mrs. Selden burst through with a "presence" that made both boys
jump. She kept up a constant tirade at her husband: he was clumsy;
he was going to break the storm sash; he was not washing the
windows clean; etc. All he was heard to say in reply was a patient,
"Yes, Martha; no, Martha."

Listening to the voices that scarcely could have been more
distinct if they had been talking directly into the mike, Carl's
face suddenly took on a very thoughtful look. He peeped over
the hedge at Mr. and Mrs. Selden, now sitting in their porch
swing, and then turned to Jerry.

"Didn't you say applying negative feedback corrected imperfections
in the output?" he demanded in a whisper.

"That's right, but so what?"

"Wait here. I'll be right back," Carl ordered, as he left
on a stooping run for Jerry's basement laboratory.

He was, back very shortly carrying a small extension speaker
for the tape recorder. After plugging one end of its long cord
into the external speaker jack of the recorder, Carl started
crawling with it over to the hedge. Here he set up the speaker
so that its cone pointed at the couple in the porch swing only
a few feet away. Then he, directed Jerry - by means of elaborate
motions - to rewind the tape and start it playing again. Jerry
carried out the pantomimed instructions and then crawled over
to his chum.

"Those birds in the hedge certainly are happy today," Mr.
Selden remarked, when the first part of the tape started playing.
In a few minutes, Mrs. Selden's voice issued through the speaker.

"Where can that woman with such a mean voice be?" Mrs. Selden
wanted to know, as she listened to the constant scolding. "If
I were her husband I'd tell her off - why, Jim, that sounds
like your voice!"

As she continued to listen, a slow flush crept over the face
of Mrs. Selden. In the beginning, she had not recognized her
own voice; but the familiar words and phrases soon left no doubt
in her mind as to who the speaker was. She turned to her husband
- whose face was wearing a look that was apprehensive, embarrassed,
and reassuring all at once - and looking at him with eyes brimming
with tears, she said gently, "Jim, I never realized I sounded
like that. I don't see how you put up with me."

"Don't say that, Martha," he replied gently, as he placed
an arm about her quaking shoulders. "I don't really mind at
all. I know you don't mean it. It's just your way of talking."

"It WAS my way of talking," she corrected, snuggling against
his shoulder. "As long as I live, I'll never, never talk to
you like that again."

As she finished speaking, she lifted a tearstained face to
her husband's, and the boys beat a hasty, wriggling retreat
to Jerry's basement, carrying the extension speaker with them.

"Say," Carl demanded, "are you sure that was negative feedback
we were using on Mrs. Selden?"

"It must have been," Jerry said, with a broad grin. "You
heard for yourself that it was going to improve her performance.
Why do you ask?"

"Well, it looked to me as if they were about to break into
osculation when we left, and I thought you said only positive
feedback caused -"

He was not able to finish because Jerry" who hated puns,
flipped a loop of the extension speaker cord over his chum's
neck and pulled the ends taut.

Posted July 15, 2015

Carl & Jerry: Their Complete Adventures is now available. "From
1954 through 1964, Popular Electronics published 119 adventures of Carl Anderson and Jerry Bishop, two teen boys with
a passion for electronics and a knack for getting into and out of trouble with haywire lashups built
in Jerry's basement. Better still, the boys explained how it all worked, and in doing so, launched countless
young people into careers in science and technology. Now, for the first time ever, the full run of Carl
and Jerry yarns by John T. Frye are available again, in five authorized anthologies that include the
full text and all illustrations."